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Video transcript

let's say you had a circuit here and you had a battery with a voltage V and there were resistors one resistor 2 and resistor 3 up here and there's current flowing through here what if you wanted to experimentally measure the voltage across some of these elements you'd have to use a voltmeter voltmeter looks like this so a circle with a V in it is the symbol we use for a voltmeter how do you use it you take that voltmeter you bring it over to here I can't plug it in the circuit like that what I do is I take the leads of the voltmeter and I just connect them to either side of the circuit element that I want to determine the voltage cross so if I do this I connect those leads right here this voltmeter will tell me the voltage across R 3 or take the voltmeter put it over here and if I connect the leads across R 1 in parallel notice I'm hooking up the voltmeter in parallel voltmeters you always hook up in parallel this now will tell me the voltage across R 1 and if I wanted to make sure my battery was functioning correctly I could take my voltmeter and I can hook up the leads across the positive and negative terminals of the battery and see if the voltage across the battery is what I think it is that's how you use a voltmeter always hooked up in parallel but if I wanted to measure the current I don't use a voltmeter I use an ammeter and for an ammeter you do not hook up an ammeter in parallel with the element you're trying to measure you'll probably blow out the ammeter I've done it a few times it's embarrassing don't hook up the ammeter in parallel tell you why in a minute but what you have to do is hook it up in series so if I wanted to know the current going through r3 I could just stick this ammeter right in here one lead would plug into one side of the ammeter the other lead would plug into the other side this current would have to flow straight through the ammeter and this is telling me how much current goes through r3 it doesn't matter which side I put it on the current going into r3 will equal the current going out so you can put it over here too but it's got to be hooked up in series so you have to disconnect it's kind of a pain to hook up an ammeter sometimes you have to disconnect something here then connect that connection to the one side of the ammeter connect to the other side of the ammeter for a voltmeter you didn't have to do that for a voltmeter voltmeter just kept it out here just touch those leads wherever you needed to touch them but for an ammeter you have to break the circuit to let this ammeter in but I can move it wherever I want I can put it down here that tells me the current in this strip again and meters always hooked up in series with the element that you're trying to measure so this ammeter position will let me measure the current that's flowing through the battery but why is the voltmeter always hooked up in parallel and the ammeter always hooked up in series we want the ammeter to be hooked up in series because we want to measure the current through a line in the circuit we want to measure the current flowing through this resistor so if we want to measure the current flowing through something we need to make sure that the current flows through our ammeter and that's how we get our reading because of this people design a meters with very little resistance an ammeter has very little resistance and the reason is if you took this ammeter and it had a big resistance and you stuck it in here you'd be changing how much current flowed through this part of the circuit we don't want to do that whenever we measure something we don't want to disturb it so when I stick my ammeter in here I don't want to disturb how much current was going through here I wanted to know how much current flows without my ammeter being in there so when I put my ammeter in there it better have very little effect on this circuit that's why we make this ammeter have a very small resistance and that's also why you can't hook this ammeter up in parallel because if you did look what would happen this is why it's bad if I took this ammeter and I hooked it up right here and I hooked it the other side up right here look what the currents going to do I've got current flowing through here current comes this way goes this way reaches this fork in the road and it's got a choice you can go to the left or flow up through here and go through r3 or flow through my ammeter but my ammeter has very little resistance I mean small maybe on the order of a Milla M so all of this current that's flowing through here all this currents going to choose to go through my ammeter is going to just skip all those resistors forget that it just goes through the ammeter if you've got a normal-sized voltage maybe 9 volts 3 volt hooked up to a Milla you're going to burn out your ammeter there's usually a fuse in here because they know people are going to hook it up wrong I've done that and you burn out a fuse you got to go replace the fuse and to pain so don't hook up your ammeter in parallel what about voltmeters why do we hook those up in parallel well a volt meter's hooked up in parallel because we want to know the voltage across a circuit element so on either side voltage remember is defined to be the difference between electric potential at two points in space it makes no sense to ask what's the voltage through a certain point in a circuit you can ask what current flows through that point in the circuit but asking what the voltage is at a particular point in a circuit makes no sense the only thing that would make sense is asking what's the voltage across two points in a circuit so I can ask what's the voltage between at this point and that point that makes sense or I can ask what's the voltage between this point and that point that makes sense but asking what's the voltage at a point or through a point makes no sense that's what current is current flows through a point voltages across two points the difference in electric potential between two points that's why we hook up volt meters in parallel and because we hook up volt meters in parallel volt meters have to have a huge resistance sometimes on the order of hundreds of thousands of ohms or even millions of ohms so this can be big big number of ohms and the reason is think about it again our key idea is that we don't want to disturb the thing we're measuring I'm measuring the voltage across this resistor if I were to hook up a voltmeter with very little resistance I just told you what would happen this current that's flowing out of the battery but I'll try to go through this voltmeter it would not only would it try to mess up the voltmeter but that's current that's not flowing through our three anymore and so I wouldn't get a correct reading for the voltage through our three so we want to make sure our voltmeter has a big resistance so that yes I mean technically a very very small amount of current maybe a milliamp will flow through this voltmeter because it's got to take a reading but we want a small amount as possible because we want to keep this current flowing through our three the same as it was before we were measuring it because I know V equals IR and if I can measure this voltage across here I want to make sure the currents the same or I won't be getting an accurate measurement for the voltage you could ask what would happen if we did hook up the meter in series instead of parallel voltmeters have a huge resistance so if I stuck that here the volt meter has a huge resistance you wouldn't break it it's just that think about what the currents going to do current comes out of this battery it's got a choice it can go up here through r3 and the voltmeter or through r1 and r2 I said the voltmeter has hundreds of thousands even millions of ohms so this current is all going to go this way forget that it's going to skip this entirely if you hook up a voltmeter in series instead of in parallel you just kill off any current through this portion of the circuit that the voltmeter was hooked up in you probably won't break it so it's not as delicate as the ammeter but you still mess up your measurement because it wasn't designed to be used that way so remember volt meters are hooked up in parallel to the circuit element that you want to determine the voltage across but a meters are connected in series to the circuit element that you want to measure and if you're sitting there thinking oh I'm never going to hook up my ammeter in parallel whoo I'm dumb do you think I am well got to be careful because most multimeters are both voltmeters and ammeters depending on where you set the dial so if you're sitting there all day measuring current with your ammeter setting everything's going well and then you go to measure a voltage but you forget to switch the dial to volts instead of amps you'll be hooking up an ammeter in parallel erroneously that's what happened to me don't let it happen to you check the dial on your multimeter make sure it's on the function that you want it to be so you don't burn out a fuse